Key Takeaways & Executive Findings
- •• • At 300 °C, 0.5Rh/Al2O3 yields <5% N2O conversion, while 0.5Rh-4Ca/Al2O3 achieves 50% conversion, demonstrating a >10-fold improvement in low-temperature activity, critical for reducing energy input in industrial abatement. • • CaO doping enhances water resistance, a key operational advantage for treating humid industrial streams where conventional catalysts suffer deactivation. • • The 0.5Rh-4Ca/Al2O3 catalyst maintains stable conversion over 40 h on stream, indicating robust long-term durability for continuous industrial operation. • • The presence of oxygen has negligible effect on catalytic activity, simplifying process design by eliminating the need for oxygen removal upstream.
Abstract
The direct catalytic decomposition of the greenhouse gas N2O into nitrogen and oxygen holds significant environmental importance. In this study, CaO-modified Rh-based catalysts were prepared via a stepwise impregnation method. The catalyst structures were characterized, and their catalytic performances for N2O decomposition were investigated. The characterization results demonstrate that CaO doping significantly enhances catalytic activity and water resistance. Specifically, the 0.5Rh/Al2O3 catalyst exhibited a N2O conversion below 5% at 300 °C, whereas the 0.5Rh-4Ca/Al2O3 catalyst achieved 50% conversion under identical conditions, indicating markedly improved low-temperature activity. Compared to the undoped catalyst (0.5Rh/Al2O3), CaO doping (0.5Rh-xCa/Al2O3) strengthens metal-support interactions, improves Rh dispersion on the support surface, and modulates the electron density around Rh, thereby substantially boosting N2O decomposition capability. The catalyst also demonstrated excellent stability, maintaining essentially constant conversion over 40 h of reaction. These findings underscore the potential of alkaline earth metal oxide doping as an effective strategy for designing high-performance noble metal catalysts for N2O abatement.
1. Introduction
Industrial processes such as adipic acid and nitric acid production, along with fossil fuel combustion and vehicle emissions, release substantial quantities of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance. Direct catalytic decomposition of N2O into benign nitrogen and oxygen offers a compelling end-of-pipe solution. However, existing catalyst families—metal oxides, zeolites, and noble metals—face critical bottlenecks: metal oxides often require high operating temperatures (e.g., LaCoO3 achieving 50% conversion only at 467 °C), zeolites suffer from poor hydrothermal stability (e.g., Cu-APSO-34 reaching only 50% conversion at 550 °C), and noble metal catalysts, while active at lower temperatures, are hindered by high cost and poor metal dispersion.
This study addresses these limitations by doping Rh/Al2O3 with alkaline earth metal oxides, specifically CaO, via a stepwise impregnation method. The strategic addition of CaO enhances metal-support interactions, improves Rh dispersion, and modulates the electronic environment around Rh, leading to significantly enhanced low-temperature activity and water resistance. The resulting 0.5Rh-4Ca/Al2O3 catalyst achieves 50% N2O conversion at 300 °C—a temperature substantially lower than that required by many conventional catalysts—and maintains stable performance over 40 hours, offering a promising pathway for energy-efficient and durable N2O abatement in industrial settings.
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AN Haomin, BAO Shidong, ZHOU Liusong, LI Haitao, ZHENG Shourong (2026). CaO-Doped Rh/Al2O3 as a Highly Effective Catalyst for Catalytic N2O Decomposition. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2024042404
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Frequently Asked Questions
What is the specific role of CaO in enhancing the catalytic activity of Rh/Al2O3 for N2O decomposition?
CaO doping strengthens metal-support interactions, improves Rh dispersion on the γ-Al2O3 support, and modulates the electron density around Rh, collectively enhancing the intrinsic activity for N2O decomposition. This is evidenced by the significant increase in conversion from <5% to 50% at 300 °C upon CaO addition.
How does the 0.5Rh-4Ca/Al2O3 catalyst perform under humid conditions, and what implications does this have for industrial application?
The CaO-doped catalyst exhibits improved water resistance compared to undoped Rh/Al2O3, maintaining higher activity in the presence of water vapor. This is critical for treating industrial flue gases that typically contain moisture, where conventional catalysts often deactivate due to competitive adsorption or structural degradation.
What is the long-term stability of the 0.5Rh-4Ca/Al2O3 catalyst under reaction conditions?
The catalyst demonstrates excellent stability, with N2O conversion remaining essentially unchanged over 40 hours of continuous operation. This long-term durability is essential for practical applications to minimize downtime and replacement costs.
Does the presence of oxygen in the feed gas affect the catalytic performance?
No, the presence of oxygen has negligible effect on the catalytic activity of the CaO-doped Rh/Al2O3 catalyst. This is advantageous because it eliminates the need for oxygen removal upstream, simplifying process design and reducing operational costs.
What is the significance of the low-temperature activity (50% conversion at 300 °C) compared to other catalyst systems?
Achieving 50% N2O conversion at 300 °C is notably lower than the temperatures required by many metal oxide (e.g., 467 °C for LaCoO3) and zeolite (e.g., 550 °C for Cu-APSO-34) catalysts. This lower operating temperature translates to significant energy savings and reduced greenhouse gas emissions from the abatement process itself, making the CaO-doped Rh/Al2O3 catalyst a more sustainable option.
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